Potassium channels are found in a wide variety of animal cells such as nervous, muscular, glandular, immune or epithelial tissue. The channels regulating these currents open and allow the escape of potassium under certain conditions. The outward flow of potassium ions upon opening of these channels makes the interior of the cell more negative, counteracting depolarizing voltages applied to the cell. These channels are regulated, e.g., by calcium sensitivity, voltage-gating, and ATP-sensitivity.
The Drosophila Slo1 gene encodes a calcium-activated potassium channel present in both neurons and muscle (Elkins et al., Proc. Natl. Acad. Sci U.S.A. 83:8415 (1986); Atkinson et al., Science 253:551 (1991); and Adelman et al., Neuron 9:209 (1992)). Mammalian homologs of dSlo1 were cloned and found to be “Maxi” or BK (large conductance) channel types, as the single channel conductance was 272 pS with symmetrical potassium concentrations. Slo1 channels cloned from mouse and human show strong conservation of sequence and functional properties (Butler et al., Science 261:221-224 (1993); Dworetzky et al., Brain Res. Mol. Brain Res. 27:189-193 (1994); Tseng-Crank et al., Neuron 13:1315-1330 (1994); McCobb et al., Am. J. Physiol. 269:H767-H777 (1995); and Wallner et al., Rec. Chan. 3:185-199 (1995)). One proposed role of the Slo1 channel is to provide negative feedback for the entry of calcium into cells via voltage-dependent calcium channels. Perhaps because of the versatility of this mechanism, Slo1 channels are expressed in many tissues, including brain, skeletal and smooth muscle, auditory hair cells, pancreas, and adrenal gland (Marty, Nature 291:497-500 (1981); Pallotta et al., Nature 293:471-474 (1981); Petersen & Mauryama, Nature 307:693-696 (1984); Tabcharani & Misler, Biochim. Biophys. Acta. 982:62-72 (1990); Neely & Lingle, J. Physiol. 453:97-131 (1992)). In these tissues, Slo1 channels are involved in diverse functions such as regulating arteriolar smooth muscle tone (Brayden & Nelson, Science 256:532-535 (1992)), tuning of hair cell frequency (Fuchs, Curr. Op. Neurobiol. 2:457-461 (1992); Wu et al., Prog. Biophys. Mol. Bio. 63:131-158 (1996)), and modulation of transmitter release at nerve terminals (Robitaille & Charlton, J. Neurosci. 12:297-305 (1995); Knaus et al., J. Neurosci. 16:955-963 (1996)), all situations in which both membrane potential and intracellular calcium are critical factors. While numerous family members of every type of voltage-gated K+ channel have been found, to date the Slo1 channel has remained the sole functionally characterized representative of the Slo family (Wei et al., Neuropharmacology 35:805-829 (1996)).
Spermatocytes require proteins tailored to fulfill roles unique to the process of germ cell development and fertilization. Cellular signaling in spermatic cells is tightly regulated to prevent inappropriate activation of the irreversible steps that prepare the sperm to fertilize the oocyte. Many of these steps are triggered and coordinated by changes in membrane potential and intracellular Ca2+ concentration and pH. Between mating and fertilization, sperm undergo capacitation, a process which later enables them to fertilize the oocyte. Capacitation involves an increase in cytosolic pH (pHi), which promotes metabolic and swimming activity (Babcock et al., Proc. Natl. Acad. Sci. USA 80:1327-1331 (1983); Babcock & Pfeiffer, J. Biol. Chem. 262:15041-15047 (1987); Vredenburgh-Wilberg & Parrish, Mol. Reprod. Dev. 40:490-502 (1995)). This increase in pHi is accompanied by changes in membrane potential and a rise in cytoplasmic Ca2+, which trigger the acrosome reaction upon contact with the oocyte (Arnoult et al., J. Cell Biol. 134:637-645 (1996); Florman, Dev. Biol. 165:152-164 (1994)). Because of the central importance of these events in development, many efforts have been made to identify the specific proteins, including ion channels, which regulate spermatic function. In particular, there have been reports of channels present in spermatocytes and spermatids that have been proposed to play central roles in these reactions (Cook & Babcock, J. Biol. Chem. 268:22402-22407 (1993), including voltage dependent calcium channels (Florman et al., Dev. Biol. 152:304-214 (1992); Arnoult et al., Proc. Natl. Acad. Sci. USA 93:13004-13009 (1996); Lievano et al., FEBS Lett. 388:150-154 (1996); Santi et al., Am. J Physiol. 271:C1583-C1593 (1996)). Apart from a cyclic nucleotide gated channel, however, few of these channels have been directly cloned from testis (Weyand et al., Nature 368:859-863 (1994)).